Method and apparatus for reselecting a cell in heterogeneous networks in a wireless communication system

ABSTRACT

The present disclosure provides a method for cell reselection in a wireless communication system in which various radio access technologies (RATs) coexist with each other. In a wireless communication system in which heterogeneous networks coexist with each other, the method of cell reselection between heterogeneous networks for a user equipment may include: receiving a system information block (SIB) containing cell reselection parameters from a corresponding base station; checking whether cell reselection parameters based on a cell selection quality value (Squal) are configured in the received SIB; and performing, when cell reselection parameter based on Squal are not configured, cell reselection based on a cell selection receive level value (Srxlev). The present disclosure may prevent the user equipment from performing unnecessary cell reselection.

TECHNICAL FIELD

The present disclosure relates to a wireless communication system. Moreparticularly, the present disclosure relates to a method for cellreselection in a system in which Long Term Evolution (LTE), UniversalMobile Telecommunications System (UMTS), Global System for MobileCommunications (GSM) and CDMA2000 coexist with each other.

BACKGROUND ART

Recent advances in wireless communication technologies have causedrepeated evolution of communication systems technologies. As such,network operators have to manage networks in which GSM (secondgeneration mobile communication), UMTS and CDMA2000 (third generationmobile communication), and LTE (fourth generation mobile communication)coexist with each other. This has necessitated mobility support betweenheterogeneous wireless communication technologies, wirelesscommunication systems, or Radio Access Technologies (RATs).

FIG. 1 illustrates the architecture of the LTE system, to which thepresent disclosure is applied.

Referring to FIG. 1, the radio access network of the LTE system iscomposed of Evolved Node Bs (ENB, Node B or base station) 105, 110, 115and 120, Mobility Management Entity (MME) 125, and Serving Gateway(S-GW) 130. A user equipment (UE or terminal) 135 may connect to anexternal network through the ENBs 105 to 120 and the S-GW 130.

In FIG. 1, the ENBs 105 to 120 correspond to Node Bs of the existingUMTS system. The ENB is connected to the UE 135 through a radio channel,and may perform more complex functions in comparison to the existingNode B. In the LTE system, as all user traffic including real-timeservices like Voice over IP (VoIP) services is served by sharedchannels, an entity is needed to perform scheduling on the basis ofstatus information collected from UEs such as information on bufferstates, available transmit power and channels. Each of the ENBs 105 to120 performs this scheduling function. In most cases, a single ENBcontrols multiple cells. To achieve a data rate of 100 Mbps, the LTEsystem utilizes Orthogonal Frequency Division Multiplexing (OFDM) in,for example, a 20 MHz bandwidth as radio access technology. Adaptivemodulation and coding (AMC) is employed to determine the modulationscheme and channel coding rate according to UE channel states. The S-GW130 provides data bearers, and creates and releases a data bearer undercontrol of the MME 125. The MME 125 performs various control functionsincluding UE mobility management and is connected to multiple ENBs.

FIG. 2 illustrates a hierarchy of wireless protocols in the LTE system,to which the present disclosure is applied.

Referring to FIG. 2, for a UE and ENB in the LTE system, the wirelessprotocol stack is composed of Packet Data Convergence Protocol (PDCP)205 or 240, Radio Link Control (RLC) 210 or 235, Medium Access Control(MAC) 215 or 230, and a physical layer (PHY) 220 or 225. The PDCP 205 or240 performs compression and decompression of IP headers. The RLC 210 or235 reconfigures PDCP PDUs (Protocol Data Unit) to a suitable size. TheMAC 215 or 230 is connected to multiple RLC layer entities in the sameUE, and multiplexes RLC PDUs into MAC PDUs or demultiplexes MAC PDUsinto RLC PDUs. The physical layer 220 or 225 converts higher layer datainto OFDM symbols by means of channel coding and modulation andtransmits the OFDM symbols through a wireless channel, or converts OFDMsymbols received through a wireless channel into higher layer data bymeans of demodulation and channel decoding and forwards the data tohigher layers. For additional error correction, hybrid ARQ (HARQ) isused in the physical layer, and the receiving end sends 1-bit HARQACK/NACK information indicating whether a packet transmitted by thetransmitting end is received. Downlink HARQ ACK/NACK information as touplink transmission may be sent through Physical Hybrid-ARQ IndicatorChannel (PHICH), and uplink HARQ ACK/NACK information as to downlinktransmission may be sent through Physical Uplink Control Channel (PUCCH)or Physical Uplink Shared Channel (PUSCH).

Meanwhile, an LTE UE in the idle mode may perform cell reselection owingto movement or the like. For movement between cells of different(heterogeneous) RATs (e.g. GSM and UMTS), the LTE UE may use a cellselection receive level value (referred to as Srxlev) in a form ofreceived signal strength indication and a cell selection quality value(referred to as Squal) in a form of received signal quality indicationaccording to versions.

Srxlev and Squal are computed by Equation 1 given below.

Equation 1

Srxlev=Q_(rxlevmeas)−(Q_(rxlevmin)+Q_(rxlevminoffset))−Pcompensation

Squal=Q_(qualmeas)−(Q_(qualmin)+Q_(qualminoffset))

Here, Q_(rxlevmeas) indicates measured received signal strength andQ_(qualmeas) indicates measured received signal quality. Q_(rxlevmin)indicates minimum power required for operation and Q_(qualmin) indicatesminimum quality required for operation. Q_(rxlevminoffset) indicatespower offset for base stations with a higher priority andQ_(qualminoffset) indicates quality offset for base stations with ahigher priority. Pcompensation indicates a correction parameter set foruplink transmission power of the UE.

The Srxlev value is computed based on Reference Signal Received Power(RSRP) in LTE and is computed based on Received Signal Code Power (RSCP)in UMTS. The Squal value is computed based on Reference Signal ReceivedQuality (RSRQ) in LTE and is computed based on the value of Ec/No inUMTS (the received energy per chip (Ec) of the pilot channel divided bythe total noise power density (No)).

When multiple RATs (e.g. GSM, UMTS and LTE) coexist with each other asdescribed above, a UE may have to perform inter-RAT cell reselection.Here, as different RATs employ different communication schemes, the UEmay have to use different cell reselection criteria. For example, forcell reselection, only Srxlev is used in GSM, CDMA2000, UMTS and theearly LTE system (Release 8 or Rel-8). However, Srxlev and Squal may beused in the recent LTE system (from Release 9 or Rel-9).

This is described in more detail below in connection with the LTEsystem.

At inter-RAT cell reselection based on Srxlev in LTE Rel-8, whenS_(nonServingCell,x) of a cell on evaluated frequency is greater thanThresh_(x,high) during a time interval Treselection_(RAT), cellreselection to the cell is performed. Here, S_(nonServingCell,x) is theSrxlev value of a cell being measured, and the values Thresh_(x,high)and Treselection_(RAT) are transmitted by the LTE base station throughpreset SIBs or the like. For example, Thresh_(x,high) andTreselection_(RAT) values for UMTS are transmitted through SIB6;Thresh_(x,high) and Treselection_(RAT) values for GSM are transmittedthrough SIB7; and Thresh_(x,high) and Treselection_(RAT) values forCDMA2000 are transmitted through SIB8.

However, in LTE Rel-9 and later, cell reselection may be performed onthe basis of Srxlev and Squal according to circumstances. That is, inLTE Rel-9 and later, when the threshServingLowQ value is providedthrough SIB3, cell reselection is performed based on Squal. Otherwise,cell reselection is performed based on Srxlev. Specifically, in theevent that the threshServingLowQ value is provided through SIB3, cellreselection to a UMTS cell is performed when Squal is greater thanThresh_(X,HighQ) during a time interval Treselection_(RAT); and cellreselection to a GSM or CDMA2000 cell is performed when Srxlev isgreater than Thresh_(X,HighP) during a time interval Treselection_(RAT).In the event that the threshServingLowQ value is not provided throughSIB3, cell reselection to a UMTS, GSM or CDMA2000 cell is performed whenSrxlev is greater than Thresh_(X,HighP) during a time intervalTreselection_(RAT). Treselection_(RAT), Thresh_(X,HighQ) andThresh_(X,HighP) values are transmitted through preset SIBs. Morespecifically, Treselection_(RAT), Thresh_(X,HighQ) and Thresh_(X,HighP)values for UMTS are transmitted through SIB6; Treselection_(RAT) andThresh_(X,HighP) values for GSM are transmitted through SIB7; andTreselection_(RAT) and Thresh_(X,HighP) values for CDMA2000 aretransmitted through SIB8.

Accordingly, up to LTE Rel-8, inter-RAT cell reselection may beperformed solely on the basis of Srxlev. However, when a latest LTEsystem (LTE Rel-9 or later) coexists with existing 2G and 3G systems,the following problem may arise. That is, a UE may perform inter-RATcell reselection based on Srxlev while remaining in a GSM, CDMA2000 orUMTS cell, and perform inter-RAT cell reselection based on Squal whileremaining in an LTE cell. In this case, the UE in the LTE cell mayperform Squal-based cell reselection to a UMTS cell owing to a low Squalvalue. Immediately thereafter, the UE in the UMTS cell may performSrxlev-based cell reselection to an LTE cell owing to a low Srxlevvalue. Such a ping-pong effect should be resolved.

FIG. 3 illustrates a problem that may arise at cell reselection whenmultiple RATs coexist with each other.

In FIG. 3, the UE 301 capable of supporting LTE, UMTS and GSMcommunication remains in an LTE base station 303 at time T1. Atoperation 311, the UE supporting LTE Rel-9 performs Squal-based cellreselection to a UMTS cell. At operation 313, the UE remaining in theUMTS cell performs cell reselection to a GSM cell using a permittedmeasurement scheme. As the UE supports GSM Rel-6, the UE cannot performSqual-based cell reselection in the GSM cell. At operation 315, the UEin the GSM cell performs Srxlev-based cell reselection back to the LTEcell. After returning to the LTE base station, at operation 317, the UEperforms Squal-based cell reselection to the UMTS cell. At operation319, the UE in the UMTS cell performs cell reselection to the GSM cellusing a permitted measurement scheme. In this way, inter-RAT cellreselection using different schemes in different systems may cause aping-pong effect.

DISCLOSURE OF INVENTION Technical Problem

Aspects of the present disclosure are to address the above mentionedproblems. Accordingly, an aspect of the present disclosure is to providea method and apparatus for cell reselection that enable reselection of acell suitable for communication without unnecessary cell reselection atinter-RAT cell reselection in a wireless mobile communication system inwhich heterogeneous networks coexist with each other.

Solution to Problem

In a wireless communication system in which heterogeneous networkscoexist with each other, to enable reselection of a cell suitable forcommunication without unnecessary cell reselection, for a newest userequipment (Rel-9 or later), the present disclosure proposes a cellreselection method described below. For cell reselection by a userequipment (UE) remaining in an LTE cell,

-   -   among system information blocks (SIBs) broadcast from the LTE        cell, when Squal-based cell reselection parameters are        configured in an SIB containing information on a RAT acting as a        target for cell reselection (i.e. Squal measurement is        configured for cell reselection to the target RAT), the UE        performs cell reselection to the target RAT on the basis of        Squal measurement results only if the version of the target RAT        implemented internally to the UE is higher than or equal to the        LTE version of the UE, and performs cell reselection based on        Srxlev measurement results if otherwise;    -   the UE performs cell reselection to a target RAT for which        Squal-based cell reselection parameters are configured on the        basis of Squal measurement results only if the target RAT also        supports Squal-based cell reselection, and performs cell        reselection based on Srxlev measurement results if otherwise;        and    -   among SIBs broadcast from the LTE cell, when Squal-based cell        reselection parameters are configured in a SIB containing        information on a RAT acting as a target for cell reselection        (i.e. Squal measurement is configured for cell reselection to        the target RAT), the UE performs cell reselection to the target        RAT for which Squal-based cell reselection parameters are        configured on the basis of Squal measurement results only if        inter-RAT cell reselection based on Squal is supported by all        neighboring RATs that are signaled through SIBs and supported by        the UE, and performs cell reselection based on Srxlev        measurement results if otherwise.

In accordance with an aspect of the present disclosure, a method for auser equipment (UE) to conduct cell reselection between heterogeneousnetworks in a wireless communication system in which heterogeneousnetworks coexist with each other is provided. The cell reselectionmethod may include: receiving a system information block (SIB)containing cell reselection parameters from a corresponding basestation; checking whether cell reselection parameters based on a cellselection quality value (Squal) are configured in the received SIB; andperforming, when cell reselection parameter based on Squal are notconfigured, cell reselection based on a cell selection receive levelvalue (Srxlev).

In accordance with another aspect of the present disclosure, a userequipment capable of conducting cell reselection between heterogeneousnetworks in a wireless communication system in which heterogeneousnetworks coexist with each other is provided. The user equipment mayinclude: a transceiver unit to send and receive signals to and from abase station; and a control unit to control a process of receiving asystem information block (SIB) containing cell reselection parametersfrom a corresponding base station, checking whether cell reselectionparameters based on a cell selection quality value (Squal) areconfigured in the received SIB, and performing, when cell reselectionparameter based on Squal are not configured, cell reselection based on acell selection receive level value (Srxlev).

Advantageous Effects of Invention

In a feature of the present disclosure, use of the proposed methodenables sensible reselection of a cell without unnecessary cellreselection in a network wherein heterogeneous radio access technologiescoexist with each other.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates the architecture of the LTE system, to which thepresent disclosure is applied.

FIG. 2 illustrates a hierarchy of wireless protocols in the LTE system,to which the present disclosure is applied.

FIG. 3 illustrates a problem that may arise at cell reselection whenmultiple RATs coexist with each other.

FIG. 4 illustrates a first embodiment of inter-RAT cell reselection fora user equipment in the LTE system according to the present disclosure.

FIG. 5 is a flowchart illustrating operations of the user equipmentusing the first embodiment of inter-RAT cell reselection in the LTEsystem.

FIG. 6 illustrates a second embodiment of inter-RAT cell reselection fora user equipment in the LTE system according to the present disclosure.

FIG. 7 is a flowchart illustrating operations of the user equipmentusing the second embodiment of inter-RAT cell reselection in the LTEsystem.

FIG. 8 illustrates a third embodiment of inter-RAT cell reselection fora user equipment in the LTE system according to the present disclosure.

FIG. 9 is a flowchart illustrating operations of the user equipmentusing the third embodiment of inter-RAT cell reselection in the LTEsystem.

FIG. 10 is a block diagram of a user equipment according to anembodiment of the present disclosure.

FIG. 11 is a block diagram of a base station according to an embodimentof the present disclosure.

MODE FOR THE INVENTION

Hereinafter, embodiments of the present disclosure are described withreference to the accompanying drawings. Detailed descriptions ofwell-known functions and structures incorporated herein may be omittedto avoid obscuring the subject matter of the present disclosure.

The present disclosure relates to inter-RAT cell reselection for a userequipment in the LTE system.

FIG. 4 is a sequence diagram illustrating a first embodiment ofinter-RAT cell reselection for a user equipment in the LTE systemaccording to the present disclosure.

In FIG. 4, it is assumed that the UE 401 is a terminal of a latestversion (e.g. Rel-9), supports Squal-based measurement, and remains inan ENB 403 after cell selection. Namely, the UE remains in the idle modeat an LTE base station.

The UE may receive preset System Information Blocks (SIBs) from the basestation. Specifically, for cell selection/reselection to a neighboringbase station employing a different RAT (e.g. GSM or UMTS base station),at operation 411, the UE receives a SIB containing corresponding cellselection/reselection parameters.

Upon SIB reception, at operation 413, the UE checks whether Squal-basedcell reselection parameters for a target RAT (e.g. Thresh_(X,HighQ)value or corresponding value) are contained in the received SIB. Asdescribed before, among cell reselection parameters, UMTS-relatedparameters may be obtained from SIB6 and GSM-related parameters may beobtained from SIB7.

If Squal-based cell reselection parameters are not contained in thereceived SIB, at operation 417, the UE determines to conduct cellreselection based on Srxlev measurement results.

If Squal-based cell reselection parameters are contained in the receivedSIB, at operation 415, the UE examines the version of the target RATimplemented internally to the UE.

If the version of the target RAT implemented internally is lower thanthe LTE version (or the mobile communication system version) of the UE(e.g. Rel-8 or lower), at operation 417, the UE determines to conductcell reselection to the target RAT on the basis of Srxlev measurementresults. If the version of the target RAT implemented internally ishigher than or equal to the LTE version of the UE, at operation 417, theUE determines to conduct cell reselection to the target RAT on the basisof Squal measurement results. Examination of the version of a target RATimplemented internally to the UE is performed because, when the LTEversion of the UE supports cell reselection based on Squal measurementresults, inter-RAT cell reselection based on Squal measurement resultscan also be supported by the target RAT if the target RAT version ishigher than or equal to the LTE version of the UE.

Upon determining to conduct inter-RAT cell reselection based on eitherSrxlev or Squal, at operation 419, the UE performs inter-RAT cellreselection accordingly. At operations 421 and 423, the UE receivesreference signals from the target RATs. At operation 425, the UEperforms measurement according to the determination made at operation417.

FIG. 5 is a flowchart illustrating operations of a user equipment usingthe first embodiment of inter-RAT cell reselection in the LTE system.

At operation 501, the UE receives preset SIBs from the LTE base station.At operation 505, the UE obtains cell reselection parameters for aneighboring target RAT from the SIBs.

At operation 507, the UE checks whether Squal-based parameters arecontained in the cell reselection parameters for a neighboring targetRAT. If Squal-based parameters are not contained in the SIB, the UEproceeds to operation 511 at which the UE performs cell reselectionbased on Srxlev.

If Squal-based parameters are contained in the SIB, the UE proceeds tooperation 509 at which the UE checks whether the version of the targetRAT implemented internally to the UE is higher than or equal to theimplemented LTE version of the UE.

If the version of the target RAT implemented internally is lower thanthe LTE version of the UE (e.g. Rel-8 or lower), the UE proceeds tooperation 511 at which the UE performs cell reselection based on Srxlev.

If the version of the target RAT implemented internally is higher thanor equal to the LTE version of the UE, the UE proceeds to operation 513at which the UE performs cell reselection based on Squal.

Examination of the version of a target RAT implemented internally to theUE is performed because, when the LTE version of the UE supports Squal,Squal-based measurement can also be supported by the target RAT if thetarget RAT version is higher than or equal to the LTE version of the UE.

FIG. 6 is a sequence diagram illustrating a second embodiment ofinter-RAT cell reselection for a user equipment in the LTE systemaccording to the present disclosure. In FIG. 6, it is assumed that theUE 601 is a terminal of the latest version (e.g. Rel-9), supports cellreselection based on Squal measurement results, and remains in an ENB603 after cell selection. Namely, the UE remains in the idle mode at anLTE base station.

At operation 611, the UE receives a preset SIB from the LTE basestation. At operation 613, the UE checks whether Squal-based cellreselection parameters for a target RAT are configured.

If Squal-based cell reselection parameters for a target RAT areconfigured, at operation 615, the UE checks whether the target RAT to bereselected (e.g. GSM or UMTS) supports Squal-based cell reselection. Forexample, when UMTS is selected by the UE as a target RAT, the UE checkswhether it is possible to conduct Squal-based inter-RAT cell reselectionfrom the UMTS cell to the LTE cell.

If the target RAT does not support Squal-based cell reselection, atoperation 617, the UE determines to conduct Srxlev-based cellreselection. If the target RAT supports Squal-based cell reselection, atoperation 617, the UE determines to conduct Squal-based cell reselectionunder assumption that it is possible to conduct Squal-based cellreselection at the target RAT cell after cell reselection to the targetRAT cell.

Upon determining to conduct inter-RAT cell reselection based on eitherSrxlev or Squal, at operation 619, the UE performs inter-RAT cellreselection accordingly. At operations 621 and 623, the UE receivesreference signals from the target RATs. At operation 625, the UEperforms measurement according to the determination made at operation617.

FIG. 7 is a flowchart illustrating operations of the user equipmentusing the second embodiment of inter-RAT cell reselection in the LTEsystem.

At operation 701, the UE in the idle mode at an LTE cell receives presetSIBs from the LTE base station. At operation 703, the UE obtains cellreselection parameters for a neighboring target RAT from the SIBs.

At operation 705, the UE checks whether Squal-based parameters arecontained in the cell reselection parameters for a neighboring targetRAT. If Squal-based parameters are not contained in the SIB, the UEproceeds to operation 709 at which the UE performs cell reselectionbased on Srxlev.

If Squal-based parameters are contained in the SIB, the UE proceeds tooperation 707 at which the UE checks whether the target RAT supportsSqual-based cell reselection. For example, when UMTS is selected by theUE as a target RAT, the UE checks whether it is possible to conductSqual-based inter-RAT cell reselection from the UMTS cell to the LTEcell. If the target RAT does not support Squal-based cell reselection,the UE proceeds to operation 709 at which the UE determines to conductSrxlev-based cell reselection.

If the target RAT supports Squal-based cell reselection, the UE proceedsto operation 711 at which the UE determines to conduct Squal-based cellreselection under assumption that it is possible to conduct Squal-basedcell reselection at the target RAT cell after cell reselection to thetarget RAT cell.

FIG. 8 illustrates a third embodiment of inter-RAT cell reselection fora user equipment in the LTE system according to the present disclosure.

FIG. 8 is a sequence diagram illustrating a third embodiment ofinter-RAT cell reselection for a user equipment in the LTE systemaccording to the present disclosure.

In FIG. 8, it is assumed that the UE 801 is a terminal of the latestversion (e.g. Rel-9), supports cell reselection based on Squalmeasurement results, and remains in an ENB 803 after cell selection.Namely, the UE remains in the idle mode at an LTE base station.

At operation 811, the UE receives preset SIBs from the base station andobtains cell selection/reselection parameters for neighboring basestations employing different RATs (e.g. GSM and UMTS base stations).

At operation 813, the UE checks whether cell reselection parametersbased on Squal measurement results are contained as cell reselectioninformation for a neighboring RAT in a designated SIB.

If cell reselection parameters based on Squal measurement results arenot contained as cell reselection information for a neighboring RAT in adesignated SIB, at operation 817, the UE determines to conduct cellreselection to the RAT based on Srxlev measurement results.

If cell reselection parameters based on Squal measurement results arecontained as cell reselection information for a neighboring RAT in adesignated SIB, at operation 815, the UE checks whether inter-RAT cellreselection based on Squal measurement results is supported by allneighboring RATs that are signaled through designated SIBs and supportedby the UE. That is, whether all neighboring RATs support cellreselection based on Squal measurement results for the UE is examined.

If one of the neighboring RATs signaled through the designated SIBsamong all other RATs supported by the UE does not support inter-RAT cellreselection based on Squal measurement results, at operation 817, the UEdetermines to conduct cell reselection to the corresponding RAT (the RATwhose cell reselection information is configured in the designated SIB)based on Srxlev measurement results.

If all the neighboring RATs signaled through the designated SIBs amongall other RATs supported by the UE support inter-RAT cell reselectionbased on Squal measurement results, at operation 817, the UE determinesto conduct cell reselection to the corresponding RAT (the RAT whose cellreselection information is configured in the designated SIB) based onSqual measurement results. For example, if any of neighboring

RATs does not support Squal-based cell reselection, the UE does notperform cell reselection to a different RAT supporting Squal-based cellreselection.

Upon determining to conduct inter-RAT cell reselection based on eitherSrxlev or Squal, at operation 819, the UE performs inter-RAT cellreselection accordingly. At operations 821 and 823, the UE receivesreference signals from the target RAT base stations. At operation 825,the UE performs measurement according to the determination made atoperation 817.

An example for the above scheme is described. It is assumed that the UEsupports LTE, UMTS, GSM and CDMA2000 RATs. It is also assumed that cellreselection parameters based on Squal measurement results are configuredas cell reselection information for neighboring UMTS in a designatedSIB. The UE identifies cell reselection information for neighboring RATsthrough designated SIBs. When cell reselection parameters for UMTS andGSM only are configured as neighboring system information in thedesignated SIBs, the UE regards CDMA2000 as a non-neighboring RAT anddoes not consider CDMA2000 for reselection. Hence, among RATs supportedby the UE, neighboring RATs signaled through designated SIBs are GSM andUMTS. Only if the UE supports inter-RAT cell reselection based on Squalmeasurement results at both GSM and UMTS (e.g. cell reselection to LTE),the UE performs inter-RAT cell reselection based on Squal measurementresults to UMTS (the neighboring RAT whose cell reselection informationis configured based on Squal measurement results in the designated SIB).That is, the UE performs Squal-based cell reselection only when the UEsupports Squal-based measurement at both GSM and UMTS for cellreselection to LTE. If the UE does not support Squal-based cellreselection from GSM to LTE, although the UE supports Squal-based cellreselection from UMTS to LTE, the UE performs Srxlev-based cellreselection to UMTS (the neighboring RAT whose cell reselectioninformation is configured based on Squal measurement results in thedesignated SIB).

FIG. 9 is a flowchart illustrating operations of the user equipmentusing the third embodiment of inter-RAT cell reselection in the LTEsystem.

At operation 901, the UE in the idle mode at an LTE cell receivesdesignated SIBs from the LTE base station.

At operation 905, the UE obtains cell reselection parameters for aneighboring target RAT from the SIBs. At operation 907, the UE checkswhether Squal measurement-based parameters are contained in the cellreselection parameters for a neighboring target RAT. If cell reselectionparameters based on Squal measurement results are not configured in theSIB, the UE proceeds to operation 911 at which the UE performs cellreselection to the target RAT on the basis of Srxlev measurementresults.

If cell reselection parameters based on Squal measurement results areconfigured in the SIB, the UE proceeds to operation 909 at which the UEchecks whether inter-RAT cell reselection based on Squal measurementresults is supported by all neighboring RATs that are signaled throughdesignated SIBs and supported by the UE.

If one of the neighboring RATs signaled through the designated SIBsamong all other RATs supported by the UE does not support inter-RAT cellreselection based on Squal measurement results (i.e. if any of theneighboring RATs does not support cell reselection based on Squalmeasurement results), the UE proceeds to operation 911 at which the UEconducts cell reselection to the corresponding RAT (the RAT whose cellreselection information is configured based on Squal measurement resultsin the designated SIB) based on Srxlev measurement results.

If all the neighboring RATs signaled through the designated SIBs amongall other RATs supported by the UE support inter-RAT cell reselectionbased on Squal measurement results, the UE proceeds to operation 913 atwhich the UE conducts cell reselection to the corresponding RAT (the RATwhose cell reselection information is configured based on Squalmeasurement results in the designated SIB) based on Squal measurementresults.

An example for the above scheme is described. It is assumed that the UEsupports LTE, UMTS, GSM and CDMA2000 RATs. It is also assumed that cellreselection parameters based on Squal measurement results are configuredas cell reselection information for neighboring UMTS in a designated SIB(operations 905 and 907). The UE identifies cell reselection informationfor neighboring RATs through designated SIBs. When cell reselectionparameters for UMTS and GSM only are configured as neighboring systeminformation in the designated SIBs, the UE regards CDMA2000 as anon-neighboring RAT and does not consider CDMA2000 for reselection.Hence, among RATs supported by the UE, neighboring RATs signaled throughdesignated SIBs are GSM and UMTS. Only if the UE supports inter-RAT cellreselection based on Squal measurement results at both GSM and UMTS(e.g. cell reselection from GSM and UMTS to LTE) (operation 909), the UEperforms inter-RAT cell reselection to UMTS (the neighboring RAT whosecell reselection information is configured based on Squal measurementresults in the designated SIB) on the basis of Squal measurement results(operation 913). That is, the UE performs Squal-based cell reselectiononly when the UE supports Squal-based measurement at both GSM and UMTSfor cell reselection to LTE. If the UE does not support Squal-based cellreselection from GSM to LTE (operation 909), although the UE supportsSqual-based cell reselection from UMTS to LTE, the UE performsSrxlev-based cell reselection to UMTS (the neighboring RAT whose cellreselection information is configured based on Squal measurement resultsin the designated SIB) (operation 911).

FIG. 10 is a block diagram of a user equipment according to anembodiment of the present disclosure.

Referring to FIG. 10, the user equipment may include a transceiver unit1005, a control unit 1010, a mux/demux unit 1015, a control messagehandler 1030, and various higher layer units 1020 and 1025.

The transceiver unit 1005 receives data and control signals throughdownlink channels of a serving cell and sends data and control signalsthrough uplink channels. When multiple serving cells are configured, thetransceiver unit 1005 may send and receive data and control signalsthrough the multiple serving cells.

The mux/demux unit 1015 multiplexes data coming from the higher layerunits 1020 and 1025 or the control message handler 1030, anddemultiplexes data received by the transceiver unit 1005 and forwardsthe demultiplexed data to the higher layer units 1020 and 1025 or thecontrol message handler 1030.

The control message handler 1030 processes a control message receivedfrom a base station and performs a corresponding operation. For example,when DRX related parameters are received, the control message handler1030 forwards the same to the control unit 1010.

The higher layer units 1020 and 1025 may be configured on a servicebasis. The higher layer units 1020 and 1025 may process user datagenerated by service applications such as File Transfer Protocol (FTP)and Voice over Internet Protocol (VoIP) and forward the processed userdata to the mux/demux unit 1015, and delivers data coming from themux/demux unit 1015 to appropriate service applications at the higherlayer.

The control unit 1010 examines scheduling commands such as UL grantsreceived through the transceiver unit 1005, and controls the transceiverunit 1005 and the mux/demux unit 1015 so that uplink transmissions areperformed at proper points in time with appropriate transmissionresources. The control unit 1010 controls the transceiver unit 1005 forDRX operation and CSI/SRS transmission.

Particularly, in various embodiments of the present disclosure, thecontrol unit 1010 receives a SIB containing cell reselection parametersfrom a corresponding base station, and examines whether Squal-based cellreselection parameters are configured in the received SIB. IfSqual-based cell reselection parameters are not configured, the controlunit 1010 controls an operation to perform Srxlev-based cellreselection.

In the first embodiment, if Squal-based cell reselection parameters areconfigured, the control unit 1010 examines the version of a target RATto be reselected (or a target mobile communication system) implementedinternally to the UE. The control unit 1010 compares the version of thetarget RAT implemented internally with the version of a preset RAT ofthe UE. If the version of the target RAT implemented internally ishigher than or equal to the version of the preset RAT of the UE, thecontrol unit 1010 may control an operation to conduct cell reselectionbased on Squal measurement results. If the version of the target RATimplemented internally is lower than the version of the preset RAT ofthe UE, the control unit 1010 may control an operation to conduct cellreselection based on Srxlev.

In the second embodiment, if Squal-based cell reselection parameters areconfigured, the control unit 1010 checks whether a target RAT to bereselected (or a target mobile communication system) supports cellreselection based on Squal measurement results. If the target RATsupports Squal-based cell reselection, the control unit 1010 may controlan operation to perform cell reselection based on Squal measurementresults. If the target RAT does not support Squal-based cellreselection, the control unit 1010 may control an operation to performSrxlev-based cell reselection.

In the third embodiment, if Squal-based cell reselection parameters areconfigured, the control unit 1010 checks whether inter-RAT cellreselection based on Squal measurement results is supported by allneighboring RATs (or neighboring mobile communication systems) that aresignaled through designated SIBs and supported by the UE. If all theneighboring RATs support inter-RAT cell reselection based on Squalmeasurement results, the control unit 1010 may control an operation toconduct cell reselection based on Squal measurement results. If any ofthe neighboring RATs does not support inter-RAT cell reselection basedon Squal measurement results, the control unit 1010 may control anoperation to conduct Srxlev-based cell reselection.

FIG. 11 is a block diagram of a base station according to an embodimentof the present disclosure. The base station of FIG. 11 may include atransceiver unit 1105, a control unit 1110, a mux/demux unit 1120, acontrol message handler 1135, various higher layer units 1125 and 1130,and a scheduler 1115.

The transceiver unit 1105 sends data and control signals through adownlink carrier and receives data and control signals through an uplinkcarrier. When multiple carriers are configured, the transceiver unit1105 may send and receive data and control signals through the multiplecarriers.

The mux/demux unit 1120 multiplexes data coming from the higher layerunits 1125 and 1130 or the control message handler 1135, anddemultiplexes data received by the transceiver unit 1105 and forwardsthe demultiplexed data to the higher layer units 1125 and 1130, thecontrol message handler 1135 or the control unit 1110. The controlmessage handler 1135 processes a control message received from a userequipment and performs a corresponding operation, and generates acontrol message to be sent to a user equipment and forwards the controlmessage to a lower layer.

The higher layer units 1125 and 1130 may be configured on a terminal andservice basis. The higher layer units 1125 and 1130 may process userdata generated by service applications such as FTP and VoIP and forwardthe processed user data to the mux/demux unit 1120, and process datacoming from the mux/demux unit 1120 and deliver the processed data toservice applications at the higher layer.

The control unit 1110 determines CSI/SRS transmission times of userequipments and controls the transceiver unit 1105 accordingly.

The scheduler 1115 allocates transmission resources to a user equipmentat appropriate points in time in consideration of buffer states, channelstates and active time of the user equipment, and controls thetransceiver unit 1105 to send or receive a signal to or from the userequipment.

Use of the proposed method enables sensible reselection of a cellwithout unnecessary cell reselection in a network wherein heterogeneousradio access technologies coexist with each other.

While the present disclosure has been shown and described with referenceto various embodiments thereof, it should be understood by those skilledin the art that many variations and modifications of the method andapparatus described herein will still fall within the spirit and scopeof the present disclosure as defined in the appended claims and theirequivalents.

1. A method for a user equipment (UE) to conduct cell reselectionbetween heterogeneous networks in a wireless communication system inwhich heterogeneous networks coexist with each other, the methodcomprising: receiving a system information block (SIB) containing cellreselection parameters from a corresponding base station; checkingwhether cell reselection parameters based on a cell selection qualityvalue (Squal) are configured in the received SIB; and performing, whencell reselection parameter based on Squal are not configured, cellreselection based on a cell selection receive level value (Srxlev). 2.The method of claim 1, further comprising: identifying, when Squal-basedcell reselection parameters are configured, the version of a mobilecommunication system that is supported by the UE and acts as a targetfor cell reselection; comparing the version of the mobile communicationsystem acting as a target for cell reselection with the version of apreset mobile communication system supported by the UE; and performing,when the version of the mobile communication system acting as a targetfor cell reselection is higher than the version of the preset mobilecommunication system, cell reselection based on Squal measurementresults.
 3. The method of claim 2, further comprising performingSrxlev-based cell reselection when the version of the mobilecommunication system acting as a target for cell reselection is lowerthan the version of the preset mobile communication system.
 4. Themethod of claim 1, further comprising: determining, when Squal-basedcell reselection parameters are configured, whether a mobilecommunication system acting as a target for cell reselection supportscell reselection based on Squal measurement results; and performing,when the mobile communication system acting as a target for cellreselection supports cell reselection based on Squal measurementresults, cell reselection based on Squal measurement results.
 5. Themethod of claim 4, further comprising performing Srxlev-based cellreselection when the mobile communication system acting as a target forcell reselection does not support cell reselection based on Squalmeasurement results.
 6. The method of claim 1, further comprising:determining, when Squal-based cell reselection parameters areconfigured, whether cell reselection based on Squal measurement resultsis supported by all neighboring mobile communication systems that aresignaled through SIBs and supported by the UE; and performing, when cellreselection based on Squal measurement results is supported by all theneighboring mobile communication systems, cell reselection based onSqual measurement results.
 7. The method of claim 6, further comprisingperforming Srxlev-based cell reselection when any of the neighboringmobile communication systems does not support cell reselection based onSqual measurement results.
 8. A user equipment capable of conductingcell reselection between heterogeneous networks in a wirelesscommunication system in which heterogeneous networks coexist with eachother, the user equipment (UE) comprising: a transceiver unit to sendand receive signals to and from a base station; and a control unit tocontrol a process of receiving a system information block (SIB)containing cell reselection parameters from a corresponding basestation, checking whether cell reselection parameters based on a cellselection quality value (Squal) are configured in the received SIB, andperforming, when cell reselection parameter based on Squal are notconfigured, cell reselection based on a cell selection receive levelvalue (Srxlev).
 9. The user equipment of claim 8, wherein, whenSqual-based cell reselection parameters are configured, the control unitcontrols a process of identifying the version of a mobile communicationsystem that is supported by the UE and acts as a target for cellreselection, comparing the version of the mobile communication systemacting as a target for cell reselection with the version of a presetmobile communication system supported by the UE, and performing, whenthe version of the mobile communication system acting as a target forcell reselection is higher than the version of the preset mobilecommunication system, cell reselection based on Squal measurementresults.
 10. The user equipment of claim 9, wherein, when the version ofthe lo mobile communication system acting as a target for cellreselection is lower than the version of the preset mobile communicationsystem, the control unit controls an operation to perform Srxlev-basedcell reselection.
 11. The user equipment of claim 8, wherein, whenSqual-based cell reselection parameters are configured, the control unitcontrols a process of determining whether a mobile communication systemacting as a target for cell reselection supports cell reselection basedon Squal measurement results, and performing, when the mobilecommunication system acting as a target for cell reselection supportscell reselection based on Squal measurement results, cell reselectionbased on Squal measurement results.
 12. The user equipment of claim 11,wherein, when the mobile communication system acting as a target forcell reselection does not support cell reselection based on Squalmeasurement results, the control unit controls an operation to performSrxlev-based cell reselection.
 13. The user equipment of claim 8,wherein, when Squal-based cell reselection parameters are configured,the control unit controls a process of determining whether cellreselection based on Squal measurement results is supported by allneighboring mobile communication systems that are signaled through SIBsand supported by the UE, and performing, when cell reselection based onSqual measurement results is supported by all the neighboring mobilecommunication systems, cell reselection based on Squal measurementresults.
 14. The user equipment of claim 13, wherein, when any of theneighboring mobile communication systems does not support cellreselection based on Squal measurement results, the control unitcontrols an operation to perform Srxlev-based cell reselection.